The method disclosed herein, in general, relates to a process for doping the surface of a solar cell. More particularly, the method disclosed herein relates to a use of a doped film on the surface of the solar cell as a diffusion source.
Typical solar cells are made on p-type silicon wafers with n-type diffusion on a front surface to form a p-n junction. The diffused region is called an emitter, and is usually formed by phosphorus diffusion. The emitter is doped heavily in order to form one or more metal contacts as well as to obtain a low sheet resistance. Silver paste is typically used to provide a metal contact. The silver paste penetrates into the emitter when the silicon wafer is heat treated. Doping of the emitter region needs to be thick to contain the silver within the emitter region and not allow the silver to penetrate into a depletion region or a p-type region. The practical requirements of the heavy phosphorous doping in the emitter region suffer from drawbacks, for example, high surface recombination, high emitter dark current, poor lifetime of the emitter region, and poor spectral response for light absorbed in or near the emitter. Typical solar cells produced in this manner have efficiencies of about fourteen percent.
Hence, there is a long felt need for selective heavy doping of the emitter region under the metal contacts to obtain low contact resistance for the metal and to repel minority carriers from the metal contacts. There is also a need for selective light doping on the rest of the emitter region to achieve low surface recombination, low emitter dark current, higher lifetime of the emitter region, and an improved spectral response for light absorbed in the emitter region. There is this need for dual doping of a silicon wafer for increasing the efficiency of the silicon solar cells. There is also a need for selective heavy doping to form high-low junctions on the back of the semiconductor device, also referred to as back surface fields, or back contact regions on silicon solar cells.